Solid state polymerization involves the molecular weight buildup of a low molecular weight prepolymer, usually in the form of a pellet or chip or a finely divided state such as powder, by heating under vacuum or with a hot inert gas stream.
Three types of processes have been developed. These include and are known as the vacuum process, as disclosed in U.S. Pat. No. 2,534,028 to Izard, the static bed process and the fluidized bed process, both of the latter being described in U.S. Pat. No. 3,497,477. In the static bed process an inert gas is passed continuously through a bed of prepolymer pellets or chips, at a temperature of from 200.degree. C. to 300.degree. C. until the prepolymer attains an intrinsic viscosity of at least 0.5.
In the fluidized bed process an inert gas is passed through a bed of finely ground prepolymer powder at a rate sufficient to suspend the particles of powder in the gas and maintain it in suspended condition while maintaining the temperature of the gas in the fluidization zone in the range of from about 5.degree. C. to about 50.degree. C. below the melting point of the polymer. Passage of the gas and fluidization of the polymer is continued until the polymer has an intrinsic viscosity of at least 0.5. Usually the gas is passed into a second zone where fine particles of polymer are recovered from the gas and volatile materials such as glycol, water, aldehydes and other materials distilled from the polymer are separated from the gas and the gas is recirculated to the polymerization zone.
Heretofore the form of the prepolymer has more or less dictated the type of solid state polymerization process which could be employed to convert low molecular weight prepolymer to high molecular weight polymer. For example, it has generally been accepted practice to use prepolymer in the form of pellets or chips in vacuum and static bed processes and finely ground powder in fluidized bed processes. The reason for this is that experience has shown that finely ground powders tend to agglomerate in vacuum processes, resulting in slower polymerization rates and a need to regrind the final polymer. Experience has also shown that, in static bed processes finely ground powders will channel or fissure, resulting in uneven polymerization and prolonged polymerization rates. On the other hand the use of pellets or chips in fluidized bed processes is not economically feasible in view of the velocity and volume of gas needed to suspend the pellets or chips and the size of the equipment required to do this.